“Snape?’ said Black harshly, taking his eyes off Scabbers for the first time in minutes and looking up at Lupin. ‘What’s Snape got to do with it?’
‘He’s here, Sirius,’ said Lupin heavily. ‘He’s teaching here as well.’ He looked up at Harry, Ron and Hermione.
‘Professor Snape was at school with us. He fought very hard against my appointment to the Defence Against the Dark Arts job. He has been telling Dumbledore all year that I am not to be trusted. He has his reasons … you see, Sirius here played a trick on him which nearly killed him, a trick which involved me -‘
Black made a derisive noise.
‘It served him right,’ he sneered. ‘Sneaking around, trying to find out what we were up to … hoping he could get us expelled …”
-Prisoner of Azkaban, Ch. 18
Have I not noticed this before or just forgotten it? Because it seems like it should be more common knowledge, given it's canon, than it is.
Snape wasn't being nosy. He was hoping to get the Marauders expelled. A kid getting bullied four-on-one by privileged kids who, as we know from canon, hex others for fun and only ever get a slap on the wrist in the form of detention, wants to get those bullies expelled? Of course he does. He wants to catch them out doing something they would surely have to experience more serious repercussions for.
Snape was frustrated and bitter because the abuse he experienced at home from his father was continued, in a different form, at school, and by people using magic - the one ability that he had and his father didn't. He was traumatized and still experiencing trauma, presumably living in survivor mode like most abuse victims do. Sirius, of course, doesn't know this. He's justified his bullying of Snape to himself, as he and James have likely justified bullying others on a whim as just harmless fun (though I'm sure students like Bertram Aubrey, whose head they hexed to be twice its size, weren't amused - ref. in HBP ch. 24). Sirius is retaliating against Snape's attempts to get him and his friends expelled, ignoring his own role in motivating Snape to do this, and he preys on the frustration and determination he's planted in Snape with his bullying to lure him to the Whomping Willow and down the passageway.
Of course Snape goes. He's desperately looking for an egregious enough offense he can prove the Marauders guilty of to get rid of them, and finally have some peace at school. A few months later he'll sit under a tree by the lake, minding his own business, when these same boys will target him out of sheer boredom (and seeking a girl's attention) to torture and humiliate him. They'll feel untouchable then. Because Snape did discover something about the Marauders when he went to the Whomping Willow, but not what he needed to - he discovered that Lupin was a werewolf, and that he was at school under Dumbledore's protection. Sirius knew he was going to the Willow, he even warned James, which means the two of them didn't transform into animagi, and thus Snape never knew the thing he could have used against them to Dumbledore: that they were running around the countryside with a full-grown werewolf, endangering students and the local population while performing dangerous and regulated magic without permission. That Lupin had betrayed Dumbledore's trust and protective measures for a bit of fun.
It's hard to blame Lupin - a lonely kid with a condition that sets him apart from society would be hard put to reject friends of any kind, let alone ones who are willing to keep his secret, not to mention learn complex, dangerous magic to keep him company during his worst times. Of course, given that James and Sirius are written as fairly selfish, entitled, privileged kids at that age, it's likely they did this more for themselves and the thrill of it, especially if the ease with which Sirius used Lupin as an unwitting weapon against Snape is any indication. But Lupin probably doesn't see this, or want to acknowledge its implications if he does, because just having friends is more than he expected from his time at school.
So Snape discovers Lupin's secret thanks to Sirius, whose main motivation was to cause Snape harm at Lupin's expense in retaliation for Snape trying to get them expelled, yet it escapes him that this kind of callous, casual cruelty that he's capable of is the reason Snape keeps trying to do this in the first place. But Snape doesn't discover the part of the secret that could get the Marauders expelled, and instead he walks away with the understanding that they are untouchable under Dumbledore's protection. Dumbledore, meanwhile, only knows that he's protected a disabled student, because he has no idea what his friends are doing, or that he's eschewing the protections put in place for his and others' safety and they're helping him to do so.
It's hard not to see Sirius as manipulative in all this. He bullies fellow students, and goads Snape, the one he targets most, into a life threatening situation. He does this as payback for Snape refusing to cow to the abuse Sirius and his friends heap on him - because abuse is what the kind of bullying we see Sirius and James do to Snape is. And the kind of 'I'll teach you to question me' attitude Sirius is reflecting by doing so in sending Snape to the Willow is typical of abusers. He knows how to get away with it, too. Perhaps he warns James about Snape going to the Whomping Willow because James wants to tranform and get up to their usual adventures, and Sirius wants to protect him - because if Snape sees that they're animagi, he'll have something more consequential to take to the administration than hexing people in the hallway. Whatever reason he has, he tells James, who rushes to the Willow to pull Snape away from Lupin, whether for his own safety or to protect Lupin, or both. Sirius comes from a wealthy, privileged, posh family, and though he rejects their politics, he doesn't seem to reject the self-serving entitlement they have. This, unfortunately, feeds into his abusive behavior - he might justify to himself that bullying Snape is retaliating against the kind of evil he sees in his all-Slytherin, pureblood, upper class family at home, but it's still unfair to Snape and Sirius shows no reflection or self-awareness in targeting a fellow student who bears unmistakeable signs of poverty and a working class upbringing. As we see in Snape's Worst Memory, the Marauders target Snape two-to-one at the very least, and not one person in the crowd of students (which must include Slytherins) will defend him. However Sirius might justify his actions to himself, he has multiple unfair advantages, which he presses without self-interrogation.
Snape's frustration and hatred are a natural reaction to all this. He's being abused by a group of boys who have strength in numbers. They bully others too, but as Lupin says, "Snape was a special case." He's targeted more frequently and with more vitriol. His attackers only ever get a slap on the wrist, despite their frequent detentions doing nothing, it seems, to discourage their behavior, especially as James and Sirius often serve them together, so it was likely great fun for them. Snape's only hope of peace and safety is getting them expelled, and his abuse trauma from home is likely helping fuel his desperation to achieve this. He fails to do so, and despite almost getting killed, the powers that be - Dumbledore, as headmaster - betrays him (and all the other students, in his eyes), by protecting the boys who tried to kill him for trying to, in his mind, protect himself.
Snape wasn't being nosy, that's just Sirius' narrative - a self-serving one that conveniently sidesteps taking responsibility for his own role in all of this. The key words in the above quote aren't that Snape was trying find out what the Marauders were up to, it's a misdirect, the same way Lupin's idea that Snape was jealous of James' quidditch talent is. The real motivation behind Snape's actions, as we can only understand after reading The Prince's Tale, is that he wanted to get the Marauders expelled.
